Added a new rule filechk used to check when a generated file
actually is changed. If there is no actual changes the file
is left without updating the timestamp.
When building a kernel from scratch two printouts occurs:
  CHK      file-to-generate
  UPD      file-to-generate

The first line tell that kbuild checks the file, second line tell that
the file is being updated (or created).
On successive runs only the first line is printed.
Output is the same in verbose and non-verbose mode.

This replaces the former update-if-changed which has been deleted.
generate-asm-offsets.h has been renamed as well.
All users are updated in next patch.
Output when generating compile.h follow above style